In a lithography process when manufacturing a semiconductor device, a mask (reticle) that is four times in size of a pattern that is actually formed and an exposure apparatus composed of a reduction projection optical system are used for forming a fine pattern.
However, recently, with the miniaturization of a pattern, formation of a mask pattern is becoming difficult even by using a quadruple mask. Moreover, because of a physical limit of a member and a design of an optical system, a size of a pattern formable on a wafer is approaching the limit. As a Resolution Enhancement Techniques (RET) to address such a problem, a novel exposure technique such as a double patterning is proposed. This double patterning has many problems that need to be solved such as displacement that occurs when superimposing the first exposure with the second exposure, so that pattern formation is not easy. Therefore, it is desired to correctly calculate an optical image intensity distribution formed by whole image exposure in a short time for forming a desired fine pattern.